1. Field of the Invention
The invention is based on a protection method and on a protection device for detecting asynchronism during the synchronous starting of synchronous machines.
2. Background of the Invention
The present invention relates to a prior art system which is known from the Swiss house journal: Brown Review 7/8 (1983), pages 295-302. In this journal, various starting methods, selected and optimized depending on power system conditions, are specified for synchronous machines which are operated both as generators and as motors, such as pumped storage plants and gas turbine sets. During so-called fully synchronous starting, the two electrically connected salient-pole machines are already excited by static exciter devices when they are standing still. The mechanically driven generator initially supplies the motor with a stator current with a very low frequency which generates an electromagnetic torque in the motor. The motor becomes synchronized with the generator under certain specified conditions which have already been successfully used with a ratio of machine powers of 1:20. In this arrangement, the stators of both machines are electrically connected, if necessary, over long distances and with the interposition of several transformers if required. In so-called partially synchronous starting, the motor initially starts asynchronously. Excitation only occurs after the generator speed has been reached after and which the motor is synchronously run up to speed.
However, protective measure is specified for the case where the field current is too low or the turbine controller runs up the generator too fast so that the motor cannot follow. During synchronous run-up of the motor, loss of synchronism, i.e. too much slip, can occur which in turn can lead to inadmissibly high heating of the heavy-current parts of the motor and to overvoltages in the field circuit. It is therefore desirous to prevent these occurrences.